


Where The Sins Crawl

by ForeverShippingJohnlock



Series: Fellowship of Literacy Prompts [3]
Category: Kuroshitsuji | Black Butler, Undertale (Video Game)
Genre: Black Butler Undertale AU, Childhood Trauma, Gen, Psychopathology & Sociopathy, Sebastian as the role of Chara, Undertale Genocide Route, Violence, ambiguous time period, au where Ciel escapes himself when he's 10
Language: English
Status: Completed
Published: 2017-11-16
Updated: 2017-11-16
Packaged: 2019-02-03 07:43:25
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,245
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/12744009
Author URL: https://archiveofourown.org/users/ForeverShippingJohnlock/pseuds/ForeverShippingJohnlock
Summary: A ten-year-old Ciel manages to escape from his captors and seeks shelter at Mt. Ebott. When he falls into the Underground, he discovers his craving for power, and he'll do anything to get it. Even if it means giving up his soul.





	Where The Sins Crawl

**Author's Note:**

> Hello again! The prompt for this one was free theme, so I could do what I wanted and I chose an Undertale AU :) Hope you enjoy it :D

It takes Ciel weeks to perfect his breathing. Between the bouts of torturous pain and the humiliating violation, he sits in his dark cage, damp from the sweat and waste of terrified children, and holds his breath. If he's not holding his breath, he's trying to make it as shallow as he can, slowing his heart rate down until even he has trouble feeling it. He's passed out more than once from attempting such a feat, and part of him hopes he never wakes up. But then determination swells in his chest, heavy and desperate, and forcefully stamps out any notion of giving up. 

In the beginning, he had tried to connect with the other children, to use this determination he felt to help them, that perhaps they could all work together to overcome the atrocities that had befallen them. But as the days turned into weeks, and the weeks into months, he could see that it was impossible; the others' minds soon grew as fragile as their ever-weakening bodies, and hopelessness glazed their slack and vacant features. At first, Ciel had felt sympathy for them, but as time went on and he was forced to look again and again upon their helplessness, contempt soon bloomed. Ciel would be twice as strong as these pathetic children were. He would not become them. 

He would stay determined. 

Tonight is the night he puts his carefully laid plan into action. He waits until the group gathers, clad in their ceremonial robes and anonymous masks. He lays on the floor of his cage, closes his eyes, and waits. 

Time passes slowly in this twisted amphitheater. Nowhere for sunlight to slip through, not even moonlight. His world is made of shadows now, but he has long since conquered his fear of the dark.  

Only ten years of age, Ciel Phantomhive has gazed unflinching into the abyss, and the abyss has gazed back.  

Soon enough, he hears two sets of footsteps approaching. He lays still, barely breathing. The cage door opens. 

"Not another one, that's the fourth this week!" he hears one of the men say in an outraged voice.  

"Droppin' like flies, aren't they?" the other one intones. "Check his pulse, make sure he's not just unconscious." 

The first man makes an affirmative noise in his throat and grunts as he bends down and grasps Ciel by the ankles, dragging him out of the cage. Ciel stays purposefully limp, he's trained for this. He knows that this is his only chance and he will  _not_ waste it. 

He is master of his heartbeat, and doesn't even flinch as the man's hands touch his delicate throat. He plays with the line between consciousness and its opposite, as he waits for the man's verdict. 

"Don't feel anything. Real shame, too, he was a favourite o' mine," he says. The other man makes a tutting noise and agrees. 

"Mine too. Put 'im with the rest for now, we'll take care of it later," he replies. Ciel is thrown over a shoulder and carried away, further than he's been in months.  

Ciel listens intently, hearing several doors open and close as they walk through. He starts to feel lightheaded, and with his eyes already closed, it would be so easy to slip into that eagerly awaiting dark, but he stays his ground. He stays determined. 

The fifth door brings a breeze and the footsteps that once echoed against marble now crunch on gravel. The wind is crisp as it ruffles Ciel's hair, and for a moment he is awash in pure sensation. A few more steps, and then he's being thrown onto something that is soft with hard edges. Ciel continues to wait as he strains his ears, remaining still as he listens to the man's fading footsteps and then, finally, the closing of the door. As silence engulfs his surroundings, Ciel snaps his eyes open and gasps for air. 

He squints at the onslaught of light that pierces his vision. It's nearing dusk, but even the dim blue is overwhelming after so much time spent in darkness. Ciel doesn't move right away, feeling woozy from the gulped influx of oxygen coursing through to his brain. He looks around slowly, and it's only then he realizes what he'd landed on. 

He gasps and scrambles away, his knees skidding against the unforgiving gravel. He stares at a small pile of children's bodies, in various stages of greying decay. The scent wafts over to him and he nearly retches. Tearing his horrified gaze away, his eyes wildly survey his surroundings.  

The manor they have been kept in all this time looms large beside him, and all around it is nothing but miles of forest. Ciel doesn't know if he'll survive or for how long, but he does know that he'd rather die alone in the woods than here on his knees like a dog.  

So, he runs. 

It starts as a stumbling walk, his legs weak from disuse, but he steadily gains speed and runs as fast as he possibly can. His bare feet bleed as they cut into rocks and twigs, but he pays it no mind. He trips on the occasional root, but just as quickly gets up, not daring to look back. He runs until he's doubled over from the stitch in his chest, his legs aching so badly to the point of numbness. It's only then that Ciel collapses to the ground, utterly spent. 

He sits on the forest floor and pulls his knees up to his chest, hugging them in a fruitless attempt to stay warm as the night winds blow through his thin and tattered clothing. He puts his head to his knees, his eyes stinging with tears he steadfastly refuses to shed. In the first week of his captivity, he had cried more tears than he thought possible, and quickly learned that they were pointless.

When Ciel lifts his head back up again, the dim blue has slipped even more into the inevitable night sky, and he wearily gets up off the ground, knowing he has to find shelter. As he stands, his gaze lands squarely on a mountain, the shape of which he can just make out in the gloom. It doesn’t look to be all that far away, so he decides to walk there, hoping to find a cave where he can lay his head for the night.

 He doesn’t know how long it takes him, but the stars are just beginning to make their appearance as he reaches the mountain’s base. Ciel looks up, intimidated by the massive structure, if only for a moment. Not seeing anything and hoping there would be something higher up, he begins to climb.

After a short while, he can see a sort of tunnel that has worked its way into the rock. He quickens his pace and soon comes to the mouth of a cave. He breathes a relieved sigh and cautiously enters.

The darkness overwhelms him immediately. The moonlight from behind him beckons him to go back, but his determination drives him forward. He walks a few more steps, and then a few more, the mouth of the cave getting smaller and smaller in the growing distance. He goes to take another step, but his foot catches on something.

And then he’s falling.

Fear and adrenaline spike through his heart as he squeezes his eyes shut, bracing himself for the end he knows is coming. But, after what seems like far too long, he lands on something relatively soft.

Ciel lays there, hardly daring to breathe. He hesitantly opens his eyes to find himself mostly unharmed, with only a few scrapes. He puts his hand on the earth and presses it into the soft soil he finds there, bright yellow flowers littering the entire area.

He notices a tunnel leading away from his current resting place, and knows he has no other choice but to follow it. He gingerly stands and makes his way towards it. Ciel’s suspicion grows with each step he takes and, on a whim, he grabs a stick that must have made the fall with him. He carries it tightly in one white-knuckled hand, unsure what he’s preparing for but wanting to be ready.

At the end of the tunnel, a door stands bizarrely in front of him. Taking a steadying breath, he opens it and steps through. The door slams behind him, echoing in the new chamber. He can’t make out much in the cavern, but continues to make his way forward. The only thing he can see is another yellow flower in the distance. Not knowing what else to do, he walks up to it.

When the flower suddenly sprouts a smiling face and begins to talk, Ciel yelps and jumps back, brandishing his makeshift weapon. The flower chuckles a little, good-naturedly.

“Howdy! I’m Flowey, Flowey the flower! Hmm… You’re new to the Underground, aren’tcha? Golly, you must be so confused! Someone ought to teach you how things work around here. I guess little old me will have to do. Ready? Here we go!” the flower – “Flowey” – chirps happily. He barely lets Ciel get a word in, as if he’s reading from some sort of well-practiced script.

When Flowey begins to prattle on about “love” and souls, Ciel decides that he’s had just about enough. There’s something about Flowey that makes him deeply uncomfortable and distrusting; he knows from experience that even the sweetest of words can hide the most despicable of actions. So, whether out of self-preservation or simple child mischievousness, when Flowey says to catch the “friendliness pellets”, Ciel purposely avoids every single one. He smiles smugly at Flowey. He will not be subservient to anyone ever again. Least of all an insignificant – anthropomorphic or not – flower.

Flowey’s smile slips, just a little. “Hey buddy, you missed them. Let’s try this again, okay?”

The pellets fly faster, and Ciel dodges them just as accurately. Flowey shows more and more of his true colours the more Ciel avoids what he now figures is an attack – with Flowey being stupid enough to call them “bullets” rather than “friendliness pellets” at one point – until his act drops completely and his cartoon smile turns into a devilish grin that splits his entire face. Ciel only smiles, refusing to back down, triumphant in the fact that his instincts were correct. He holds up his stick higher, preparing to defend himself.

Flowey’s chirpy tone turns dark and sinister, “You know what’s going on here, don’t you? You just wanted to see me suffer. DIE.”

Suddenly a ring of bullets encircles Ciel, with no way to dodge this time. The ring closes in, nearing Ciel’s tender throat until he can feel the heat of the attack on his skin. He tilts his chin up and looks into Flowey’s cackling face, refusing to submit even at the time of his imminent demise.

So, he’s surprised when Flowey makes a strangled noise as some kind of fireball hits him, throwing him off into the darkness. The ring disappears and Ciel is staring into the kind eyes of a tall and motherly looking creature with features somehow both goat and human.

“What a terrible creature, torturing such a poor, innocent youth…”

Ciel can’t help it; he lets out a small snort at the thought of him as “innocent”. He has resigned himself to the fact that he is forever tainted, damaged goods.

The creature seems to mistake the noise for fear, as she continues, “Ah, do not be afraid, my child. I am Toriel, caretaker of the Ruins. I pass through this place every day to see if anyone has fallen down. You are the first human to come down here in a long time. Come! I will guide you through the catacombs.”

Ciel’s first instinct is to stay where he is, but he knows that it would be illogical. He doesn’t know this place – the “Underground” – and Toriel clearly does. He’ll simply take what he needs from her, and be on his way. So, he follows.

They go into the next room, and Ciel comes face-to-face with a shining beacon, looking like an earth-bound star. He cautiously walks up to it and, as if controlled by another force, he reaches his hand out to touch it. The moment his hand makes contact, pleasant warmth fills him and he feels good for the first time in a very long while. After a moment, he removes his hand and the feeling leaves as quickly as it had appeared.

Toriel leads him through the Ruins, explaining and demonstrating all the ridiculous and pointless puzzles. Ciel doesn’t pay attention, and is pleased when she leaves him to his own devices.

The first time he encounters a Froggit, he does what Toriel says and attempts to talk to it. But even after that, the Froggit attacks and Ciel has extreme difficulty dodging them, angry that Toriel’s so-called strategy hadn’t worked. With each hit he feels weaker and weaker until-

Ciel is standing beside the star, feeling as good as the first time he touched it. He’s confused, had it all been some kind of dream? Feeling brazen and with nothing to lose, he walks back to the Froggit and stands there, dodging nothing, and suddenly he’s back at the star again. This beacon somehow saves his journey’s progress. Ciel smiles a little, pleased with this development. He’d never felt invincible before.

This time when he goes back to the Froggit, he feels more prepared. He dodges quicker and with more accuracy, but chooses to fight when he can. He watches with victorious satisfaction as the creature turns to dust. He will not allow anyone to hurt him again, he’s determined about that.

Ciel continues on his way, touching save points and reducing whatever creature he comes upon to nothing but ashes. The more he triumphs over these monsters, the better he feels. He remembers what Toriel had said about mercy, but just as quickly dismisses it. No one had shown mercy to him, so why should he show mercy to them? Determination rises in his chest like the tide, and he trudges on.

He soon arrives at Toriel’s house. The moment Ciel had met this woman, he could tell she was a fool with a very obvious weak point for children in need. He played his part well, and she was only too quick to give him food and shelter and a striped sweater. No matter how motherly she seemed, Ciel didn’t trust her. He happens upon old toys and children sized shoes, and knows there is more going on here than Toriel is letting on. He is determined to find out what it is; he is not a thing to be “kept”. Not anymore.

The moment he goes downstairs is the moment he finds her secret. Moving faster than Ciel had thought her capable, she ushers him back upstairs several times. Each time, Ciel marches right back down the stairs, following her, and getting further and further down the hallway until Toriel’s mothering gaze finally turns to contempt. Ciel is sensing a pattern in his interactions with these monsters, and he tightens his grip on the toy knife he’d picked up on his explorations.

“Every human that falls down here meets the same fate. I have seen it again and again. They come. They leave. They die. You naïve child… If you leave the Ruins, they… Asgore… will kill you. I am only protecting you, do you understand?” Toriel says with a stormy expression.

Ciel looks up at her, “I don’t need your protection.”

His voice is a little hoarse from disuse, but he gets his message across loud and clear. By Toriel’s pained expression, he may as well have slapped her. Her face slides from shocked, to hurt, to sad, and settles on grim resignation. They are standing in front of large purple doors.

“You want to leave so badly? Hmph. You are just like the others. There is only one solution to this. Prove yourself. Prove to me you are strong enough to survive,” she says, blocking his way.

Ciel prepares himself for a fight, especially after learning that Toriel is what they call a “boss monster”, but he almost laughs when she’s brought to her knees after one hit. He stands over her, waiting.

“Y… you… really hate me that much?” she says, voice weak.

Ciel leans over and brings his face closer to hers and looks her in her pained eyes. “Yes.”

Her expression of betrayal is the last thing Ciel sees before she, too, turns to dust.

Ciel is hit with an influx of pure power and determination, and he’s high on the exhilarating sensation. In the back of his mind, he’s struck with the notion that someone is applauding him, pleased with his progress. It feels as if he’s not alone in his own body anymore. The thought should be disturbing, but instead, Ciel finds it oddly comforting. He’s happy that he’s made this other being proud, and he strides through the purple doors, wanting to chase this feeling.

* * *

Ciel spends a very long time exploring every inch of “Snowdin”, chasing down any and all monsters in the area. He meets character after character, fighting them and winning, watching his power increase with every blow landed. He has to put up with the antics of the two ridiculous skeleton brothers, anxious for his chance to cut their insufferable jokes short with his blade.

He bides his time as the presence inside of him grows louder and more pronounced.

_Sixteen left._

_Ten left._

_Three left._

In a deep and smooth baritone, a voice whispers to Ciel how many more creatures he needs to consume on this path he’s carved for himself, and Ciel searches until he’s eradicated them all.

He enters the town, now deserted out of fear of him. After being so afraid for so long, it feels dizzyingly good to have people be afraid of him for once. He continues and soon encounters the tall skeleton, ever the imbecilic optimist even in the face of his destruction.

 _Naïve fool,_ Ciel thinks as he watches Papyrus’ stubborn head turn to ash. He can feel the presence smiling and his own face unwittingly copies the gesture. He wonders idly how this will affect the other brother, if he’ll be too lazy to seek vengeance.

Ciel continues on his way, bolstered by power, determination, and the growing presence within him.

* * *

Ciel repeats his delightful destruction in Waterfall, Hotlands, and the Core. With his power, no monster stands a chance. Some monsters prove a welcome challenge – the fish woman, the effeminate robot – but he ends them mercilessly.

Ciel soon comes to the depressive grey of “New Home”. He knows he’s getting close, he can feel it, and it’s a feeling shared by the presence. He’s curious to meet this “Asgore” he’s been hearing so much about. He wants to ask how it feels to be the king of a kingdom brought to its knees by a child.

After walking the halls, he steps into the elevator that takes him into a yellow, shadowy corridor. The light spills in from the large windows, beaming onto the checked floor and grand pillars, looking for all the world like sunlight, an impossible feat in the Underground.

Ciel’s footsteps echo on the marble floors as he walks towards the shadow some little ways ahead. He stops just short of it.

“Heya,” Sans says. Ciel smiles; he’s been waiting for this, and so has his other self. “You’ve been busy, huh?”

Ciel doesn’t reply and toys with his knife, thinking the answer rather obvious. Sans continues.

“So, I’ve got a question for ya. Do you think even the worst person can change? That everybody can be a good person, if they just try?” He chuckles a moment, not waiting for Ciel to reply, and seemingly not expecting him to. “All right. Well, here’s a better question.” The light from Sans’ eye sockets vanishes. “Do you wanna have a bad time? ‘Cause if you take another step forward, you are _really_ not going to like what happens next.”

And so, of course, Ciel takes a step forward and the fight begins.

“It’s a beautiful day outside,” Sans intones darkly. “Birds are singing, flowers are blooming… On days like these, kids like you… Should be burning in hell.”

Ciel isn’t sure what he’s expecting but the next flurry of movement and onslaught of unpredictable attacks is not it. Ciel smiles, wiping away the trickle of blood that had leaked from his mouth after being slammed against several walls. Sans’ left eye blazes blue triumphantly.

 _Who knew the lazy trashbag had it in him?_ Ciel thinks as Sans effortlessly dodges his attacks.

 _I only wish I was there to enjoy it with you,_ Ciel’s other self thinks, giving Ciel the strength to dodge the next round of attacks.

For the first time in what seems likes ages, Ciel dies.

And then he dies again.

And again.

And _again._

Over and over, he’s brought back to the save point at the beginning of the corridor, but he remains determined. Each resurrection brings a little more knowledge about Sans’ attacks, and it’s only a matter of time.

Sure enough, it’s not long until Sans begins to tire, sweat pouring from his skull (and Ciel doesn’t have the time to ponder the physicality of that). Sans continues to threaten his “special attack”, and before long, Ciel finally reaches it. He wonders what it could be and how much worse this fight will now get.

But nothing happens.

Sans can evidently sense Ciel’s confusion as he explains, “Yep. That’s right. It’s literally nothing. And it’s not gonna be anything, either.”

He goes on to say that he’s just going to stand there until the end of time, hoping Ciel would give up. Ciel howls in frustration. This idiot is wasting their _time._ But if that’s the way it has to be… Ciel stands there and waits, staying still, determination thrumming strong within his body.

After a long while, he notices with grim delight that Sans’ eyes are beginning to droop closed. Ciel continues to wait in silence as the skeleton’s eyes finally shut and he sinks into sleep.

Ever so slowly and in silence, Ciel creeps closer and brandishes his knife, bringing it down. Sans steps out of the way, but Ciel brings it down again and this time it connects. Sans takes longer to die than the others, but he soon fades like all the rest who get in Ciel’s way.

Ciel continues to the throne room, where Asgore awaits him.

He expects some kind of fight from the king, but is sorely disappointed. Asgore is a coward, and Ciel relishes wiping out almost all his health in one fell swoop. Just as Ciel holds up his knife to finish the job, though, a ring of familiar bullets appears around Asgore and incinerates him.

Flowey pops up out of the ground, spouting nonsense about how this was all part of the plan from the beginning, in a fruitless attempt to convince Ciel that he was actually worth something, that he could help. But Ciel knows – knew right from the beginning – that Flowey is a coward, just like the rest. He’s willing to say anything to save his own skin, and Ciel thinks it’s pathetic.

So, he takes great pleasure in striding forward and beating the flower until it is nothing but a grass stain beneath Ciel’s shoe, his other self encouraging him all the while.

Suddenly, everything goes black.

Ciel squints in the darkness and sees a figure standing before him. It’s a tall, dark man with ebony hair and a pale complexion, dressed in an impeccable suit. He’s smiling calmly, his eyes burning crimson. Ciel can’t look away. The man speaks in the smooth, deep tones that Ciel recognizes.

“Greetings. My name is Sebastian. Thank you. Your power awakened me from death. At first, I was so confused. Why was I brought back to life?” He pauses. “You. With your guidance, I realized the purpose of my reincarnation. Power.” A pleasant chill runs down Ciel’s spine. “Together, we eradicated the enemy and became strong. Hp. Atk. Def. Gold. Exp. Lv. Every time a number increases, that feeling… That’s me. Now. Now, we have reached the absolute. There is nothing left for us here. Let us erase this pointless world, and move on to the next. You are a great partner, but… You still have something I want. Give it to me, and we can be together forever.”

Ciel is awash in a cacophony of contradictory feelings. Happiness and sadness. Excitement and fear. Eagerness and dread. The choice, however, is simple.

“Yes,” he whispers. Sebastian smiles, his eyes burning.

“Then, it is agreed. You will give me your soul.”

Ciel is willing to give everything he has to the source of his power, of his determination. In Ciel’s mind, it’s already his. Ciel nods his head, not trusting himself to speak in this delicate moment. Sebastian’s smile stretches unnaturally wider, and his form melts into black vapour, permeating everything in its wake and coming ever nearer to Ciel. Ciel is rooted to the spot, and before his world goes black, the voice he’s come to know so well somehow speaks from everywhere and nowhere all at once.

“Then, it is done.”

**Author's Note:**

> I'd love to know what you thought of it! Thanks for reading <3


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